1. Field of the Invention
The present invention relates to an image forming apparatus including a plurality of sheet heating devices.
2. Description of the Related Art
Hitherto, image forming apparatuses, such as copying machines and printers, have been widely put into practice by utilizing the electrophotographic process. Some electrophotographic image forming apparatuses are able to form not only a monochromatic image, but also a full-color image. Further, with increasing applications of the electrophotographic image forming apparatuses in various fields, a demand for higher image quality has been established. One of factors deciding the gloss of a full-color image, in particular, is smoothness of the surface of an output image formed on a recording sheet. Therefore, there is a strong demand to increase that smoothness.
In replying to such a demand, an image forming apparatus is proposed in which a color image is formed by transferring and fusing an image of color toner made of a thermoplastic resin onto a recording sheet coated with a transparent resin layer made of a thermoplastic resin (see, e.g., Japanese Patent Laid-Open No. 64-35452 and No. 5-216322).
As a fusing unit suitable for the above-mentioned image forming method, a belt fusing unit employing a belt is proposed (see, e.g., Japanese Patent Laid-Open No. 4-216580 and No. 4-362679). In the proposed belt fusing unit, a recording sheet carrying a not-yet-fused toner image thereon is pressed under heating by the fusing unit belt made of a heat-resistant film, and the recording sheet is cooled while it is kept in close contact with the fusing unit belt. Thus, the toner image is solidified and fixated onto the recording sheet. The recording sheet including the fixated toner image is separated from the fusing unit belt and ejected to the outside.
With that related art, the toner image is solidified together with a transparent resin layer following the surface shape of the belt by the action of the belt fusing unit into such a state that the toner image is buried in the transparent resin layer of the recording sheet. As a result, the entire surface of the recording sheet is finished to a smooth surface and a color image having a superior gloss can be obtained.
In one proposed example of the recording sheet having the transparent resin layer, the transparent resin layer contains, as a main component, a thermoplastic resin having a glass transition temperature of not higher than 358K and is formed in thickness of about 10 μm (see, e.g., Japanese Patent Laid-Open No. 2003-084477).
One known example of an image forming apparatus capable of forming an image on a recording sheet having a transparent resin layer comprises a first fusing unit and a second fusing unit, the latter being constituted by a belt fusing unit. In such an image forming apparatus, a first fusing mode and a second fusing mode are switched over depending on the type of the recording sheet. The first fusing mode is a fusing mode for an ordinary recording sheet. In the first fusing mode, the recording sheet to which a toner image has been transferred is conveyed such that it passes the first fusing unit, but it does not pass the second fusing unit. The second fusing mode is a fusing mode for a recording sheet having a transparent resin layer. In the second fusing mode, a toner image on the recording sheet having the transparent resin layer is first fused onto the recording sheet by the first fusing unit. Then, the recording sheet including the fused toner image is sent to the second fusing unit. In the second fusing unit, the toner is further fused into such a state that the toner image is buried in the transparent resin layer on the recording sheet.
Generally, a recording sheet having passed a fusing unit is in a state where heat is accumulated inside the sheet. Therefore, if the recording sheet is conveyed in the heat accumulated state through a curved conveying path, the recording sheet is curled. Also, before a toner image having been heated and pressed by the fusing unit is dried, the toner image is scraped by conveying rollers, guide ribs, etc., thus resulting in a phenomenon that scrape or friction marks appear as unevenness in gloss. In order to suppress curl of the recording sheet and the scrape marks of the toner image, a cooler is disposed to quickly cool the recording sheet immediately after it has passed the first fusing unit. In the first fusing mode, the recording sheet is quickly cooled by the cooler immediately after it has passed the first fusing unit.
In the second fusing mode, when the recording sheet (i.e., the recording sheet having the transparent resin layer) having passed the first fusing unit is quickly cooled as in the first fusing mode, the recording sheet having been temporarily cooled is heated again by the second fusing unit. To heat the temporarily cooled recording sheet again to a predetermined temperature, it is required to increase the heat capacity of the second fusing unit, to raise a level of controllable temperature, or to increase the nip pressure for fusing.
In order to increase the heat capacity of the second fusing unit or to raise a level of controllable temperature in the second fusing unit, however, the amount of electric power supplied to the second fusing unit requires to be increased. This results in an increase of power consumption in the second fusing unit. Also, when the nip pressure for fusing is increased, it is required to increase torque of a motor for driving a fusing roller or a pressing roller. This results in disadvantages that the size of a driving mechanism in the second fusing unit is increased and separation of the recording sheet from the second fusing unit is deteriorated.